the synapse
TRANSCRIPT
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Physiological anatomy
1. Structure of neuron
2. Structure of synapse
3. Types of synapses
4. Classification of synapse
Electrical events at a synapse
Inhibition at synapse
Properties of synapse
OVERVIEW
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TYPE OF
SYNAPSE
SITE OF CONTIGUITY
PART OF PRESYNAPTIC
CELL
PART OF POST SYNAPTIC
CELL
AXO-
DENDRITIC
AXON DENDRITE
AXO- SOMATIC AXON SOMA
AXO-AXONAL AXON AXON
DENDRO-
DENDRITIC
DENDRITE DENDRITE
TYPES OF SYNAPSES
CLASSIFICATION OF SYNAPSES
ANATOMICAL CLASSIFICATION
TYPE I TYPE II
Axodendritic Axosomatic
Marked thickening of subsynaptic
membrane
Non continuous slight thickening in
subsynaptic membrane
Wide synaptic cleft=30nm filled with dense
ECF
Narrow synaptic cleft=20nm with not such
dense ECF
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PHYSIOLOGICAL CLASSIFICATION
CHEMICAL SYNAPSE ELECTRICAL SYNAPSE
Transmission of impulse occurs due to liberation of
chemical mediator
Pre and post synaptic membranes come very close
to form gap junctions which act as low resistance
bridges through which ions pass with ease.
Most of the synaptic junctions Some of the junctions in lateral vestibular nucleus,
hippocampus and cerebral cortex
Signals transmitted only in one direction Signals transmitted in either directions
Sensitive to hypoxia Insensitive to hypoxia
ELECTRICAL EVENTS AT A SYNAPSE
Resting membrane potential
Release of neurotransmitters
Development of postsynaptic potential
Removal of neurotransmitter
Genesis of action potential in post synaptic neuron
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DEVELOPMENT OF POST SYNAPTIC
POTENTIAL
EXCITATORY POSTSYNAPTIC
POTENTIAL (EPSP)
INHIBITORY POST SYNAPTIC
POTENTIAL(IPSP)
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REMOVAL OF TRANSMITTER
SUBSTANCE
Diffusion of transmitter substance outside the cleft.
or
Enzymatic degradation of transmitter in the cleft. Eg:ACh
or
Active reuptake of neurotransmitter back into presynaptic terminal.
Eg:NE
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GENESIS OF ACTION POTENTIAL
Soma as an integrator
Initial segment spike
Bell Magendie law; law
of dynamic polarity.
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2. Indirect inhibition
Refractory period
Renshaw cell/negative feedback
inhibition-limit excitability.
APPLIED: Strychnine decreases
postsynaptic inhibition
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Ventromedial part of
anterior horn
PRESYNAPTIC INHIBITION
a. Reduces opening of Ca+ channels
directly.
b. Increases Cl- and/or K+ permeability
so that size of action potential
reaching excitatory ending is reduced
c. Direct inhibition of neurotransmitter
release.
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19Example=Reciprocal inhibition:
APPLIED:
1. Convulsant drug picrotoxin-
decrease presynaptic inhibition
2. Barbiturates-increase presynaptic
inhibition
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3. Occlusion 4. Subliminal fringe
Significance:
1. Pattern in peripheral nerves are usually altered as they pass through synapses on the way to brain.
E.g.: referred pain.
1. The interaction between excitatory and inhibitory influences at the synapse required for integrating
and modulating activity of nervous system.
7. Synaptic plasticity
Post tetanic potentiation
Long term potentiation
Low frequency depression
Sensitization/ facilitation
Habituation/ synaptic fatigue
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REFERENCES
1. TEXTBOOK OF PHYSIOLOGY,11TH EDITION, GUYTON AND
HALL
2. TEXTBOOK OF MEDICAL PHYSIOLOGY, INDU KHURANA
3. GANONG’S REVIEW OF MEDICAL PHYSIOLGY. 23RD
EDITION.
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